Seasonal Variability in Mercury’s Calcium Exosphere Matthew Burger Morgan State University/GESTAR Rosemary Killen (NASA/GSFC) Bill McClintock (U. Colorado/LASP)

Seasonal Variability in Mercury’s Calcium

ExosphereMatthew Burger

Morgan State University/GESTAR

Rosemary Killen (NASA/GSFC)Bill McClintock (U. Colorado/LASP)

Ronald Vervack, Jr. (JHU APL)Menelaos Sarantos (UMBC)

Tim Cassidy (U. Colorado/LASP)Aimee Merkel (U. Colorado/LASP)

Part 1• Burger et al. (2012), Modeling MESSENGER

observations of calcium in Mercury’s exosphere, JGR, E00L11, doi:10.1029/2012JE004158.

Ca Data - FlybysFan tail observations S/C roll while in Mercury’s shadow

Flyby ModelsIsotropic Ejection from the Surface

Dawn Source

Exponential Dropoff from Dawn, Equatorial Point

M3M2

•Evidence for a ~5-10º shift northward during M3

•Improved fits to Fantail and Pole data•No effect on fits to the tail•This shift has no effect on the fits to

the M1 or M2 data

Orbit 22Dawn Sourceσ = 25º

Orbits 22 - 28

•New results: Burger et al., Seasonal Variability in Mercury’s Calcium Exosphere, in prep.

Part 2

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Calcium Limb Scans

Sun

Dawn

6 (dawn)

8

10 12 (noon) 14

16

18 (dusk)

Equatorial Plane

Sun

Dawn

8 Years of Limb Scans•>162 orbits with at least 5 limb scans

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Sun

Dawn

8 Years of Limb Scans•>162 orbits with at least 5 limb scans

Fits to Limb ScansFit data from each hour angle bin with I = I0 exp(-z/H) [kR] N = I×109/g [cm-2]

I0 = intercept [kR]H = scale height [km]z = altitude [km]g = g-value [phot s-1]

Variations vs. TAAI0 vs TAA H vs TAA

TAA = 0° at perihelionTAA = 180° at aphelion

Fits at 6 am (Dawn)

Variations vs. Distance

I0 vs Distance from Sun

H vs Distance from Sun

Fits at 6 am (Dawn)

We Know What Is Happening

• All the calcium comes from a small region on the surface near the dawn, equatorial point

• Comes off very hot (T>20,000 K)

• Source size and temperature don’t change much over 8 Mercury Years

• Source strength varies

with Mercury’s

distance from the sun

Burger et al. (2012)

Dawn SourceIsotropic Source

We Don’t Know Why• Not related to the surface geology

•Source is fixed in local time and does not rotate with Mercury

• Not related to the magnetosphere (ion sputtering or electron stimulated desorption)•Magnetosphere is highly variable•Wouldn’t produce a source at dawn

• Possibly related to asymmetric impact vaporization•But not clear why only Ca shows this dawn source

• Not related to Ca freezing on the nightside and vaporizing as it moves into sunlight

Terminator Motion

I0 vs Terminator Speed

H vs Terminator Speed

Fits at 6 am (dawn)

Sun moves backwards in sky

Summary• Calcium comes off the surface very hot from a small

region near dawn•The source has been stable for eight Mercury years

• The source mechanism is still unknown•We are using numerical models of the physics that we do understand to figure out what we don’t understand.